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    <p>I like the condensing of code to</p> <pre><code>condition_string = Array.new(chapter_names.size, "chapters.name=?").join(" AND ") </code></pre> <p>and this does indeed work for a join with "OR".However the AND join will not work since this means that chapters.name in a single row of a join has to be both 'preface' and 'introduction' for example.</p> <p>An even neater way of doing my original "OR" join is</p> <pre><code> named_scope :has_chapter?, lambda { | chapter_names | {:joins =&gt; [: chapters] , :conditions =&gt; { :chapters =&gt; {:name =&gt; chapter_names}}} } </code></pre> <p>Thanks to Duplex on Rails Forum (<a href="http://railsforum.com/viewtopic.php?pid=103003#p103003" rel="nofollow noreferrer">Post link</a>) In order to achieve my original problem though DUPLEX suggested this</p> <pre><code>named_scope :has_chapters, lambda { |chapter_names| {:joins =&gt; :chapters , :conditions =&gt; { :chapters =&gt; {:name =&gt; chapter_names}}, :select =&gt; "books.*, COUNT(chapters.id) AS c_count", :group =&gt; "books.id", :having =&gt; "c_count = #{chapter_names.is_a?(Array) ? chapter_names.size : 1}" } } </code></pre> <p>This may work in some flavour of SQL but not in PostgreSQL. I had to use the following</p> <pre><code>named_scope : has_chapter?, lambda { | chapter_names | {:joins =&gt; :chapters , :conditions =&gt; { :chapters =&gt; {:name =&gt; chapter_names}}, :select =&gt; "books.* , COUNT(chapters.id)", :group =&gt; Book.column_names.collect{|column_name| "books.#{column_name}"}.join(","), :having =&gt; "COUNT(chapters.id) = #{chapter_names.is_a?(Array) ? chapter_names.size : 1}" } } </code></pre> <p>The code</p> <pre><code>Book.column_names.collect{|column_name| "books.#{column_name}"}.join(",") </code></pre> <p>is required because in PostgreSQL you can't select all columns with books.* and then GROUP BY books.* each column has to be named individually :(</p>
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    1. COThe neater way is best: :conditions=>{:chapters=>{:name=>chapter_names}}, but I also like :conditions=>["chapters.chapter_names IN (?)",chapter_names]
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